In general, brazing fluxes remove oxides and contaminants from base materials to ensure good quality brazed joints. The choice of flux depends on the base material to be used, in addition to the filler metal types, heat source, and application method. Brazing joins similar and dissimilar materials by heating them in the presence of filler metal having a liquidus above 425° C.-450° C. (˜800° F.-840° F.) and below the solidus of the base material. During brazing, filler metal flows between fitted surfaces of the joint by capillary action. The minimum temperature on the surface of the component to be brazed at which said process takes place undisturbed is the so-called working temperature. It is a characteristic quantity for the filler metal in question. Filler metals can be alloys or pure metals. In general, the heat from brazing is less damaging than the heat from welding. Additionally, brazed joints typically have higher strength than soft-soldered joints. The choice of flux plays an important role in most all brazing processes, and the use of an incorrect flux can compromise joint quality.
In order to be able to form a bond with the base metal, the molten filler metal must come into direct contact with the base metal. Oxide layers of the kind present on any engineering metal surface must thus be loosened first and removed. If brazing takes place in the air, this is achieved by covering the brazing site with fluxes in the melt flow in which the oxides dissolve, are reduced or decompose at and above the active temperature of the flux.
When heated, fluxes dissolve surface oxides and protect the cleaned surfaces from re-oxidation, transfer heat from the heat source to the joint, and remove oxidation products, allowing filler metal to contact and wet the base materials. Brazing fluxes, pastes or powders, activate at temperatures below those needed to melt filler metals. Because fluxes must be in close contact with the joint surfaces, they are liquid or gaseous at brazing temperatures. They remove only surface oxides and tarnish. Other contaminants must be removed either mechanically or chemically before brazing.
Fluxes are typically classified by form (powder, liquid, or paste), base materials and filler metals they can be used with, heat source, application method, and active temperature range. Silver brazing fluxes conain boric acid and potassium borates, combined with complex potassium fluoborate and fluoride compounds. Fluorides, up to 40% in flux content, give these fluxes their characteristically low melting points and high capacity for dissolving metal oxides. High temperature fluxes, based on boric acid and alkaline borates, sometimes, contain small additions of elemental boron or silicon dioxide to increase activity and protection.
The melting point and the effective temperature of the fluxes must be matched to the working temperature of the brazing filler metal used, whereby the flux should melt at about 50-100° C. below the working temperature of the filler metal used and become fully effective from this temperature onwards. Moreover, the molten flux should form a dense, uniform coating on the workpiece which remains intact at the required brazing temperature and for the duration of the brazing period.
Assuming pure metal surfaces, the liquid filler metal is able to spread in a thin layer on the base metal surface, wetting it. The filler metal adheres to the base metal surface by a slight alloying of the base and filler metals. The filler metal spreads out over the joint surface and, after solidifying, forms a loadable joint with the base metal.
Brazing fluxes are composed substantially of salt mixtures which, in the molten state, are capable of dissolving metal oxides. These fluxes are substantially inorganic boron compounds such as, in particular, alkali borates and fluoroborates, including boric acid, and halides such as, in particular, an alkali halide; e.g. alkali fluorides.
Per the Regulation (EC) No 1272/2008 of the European Parliament and of the Council on classification, labeling and packaging of substances and mixtures” boric acid (a component of most brazing fluxes), is classified as a reproductive toxin in the European Union. This requires special labeling, leading to an effort on the part of consumers to look for boric acid free alternatives. Suitable boric acid free brazing fluxes must be developed to maintain market share and meet consumer demand.
At least one aspect of the invention resides in the superior ability to achieve desirable flux characteristics without the presence of boric acid (H3BO3) or borax (NaB4O5(OH)4.H2O) in the flux.